Infrared spectra of the interactions of aniline and porous glass surfaces

Abstract

Infrared spectra were recorded of aniline sorbed on highly dehydroxylated, deuterated, and on fluoridated porous glass as well as on pure and boria-impregnated silica. The results suggest that two types of weak interactions involving the surface SiOH and B-OH groups occurred; the nitrogen atom of the anline was hydrogen bonded to surface OH and there was an interaction between OH groups and the n system of the aromatic ring. Some aniline chemisorbed on surface boron via the nitrogen atom of the amine group. Some aniline chemisorbed dissociatively to form secondary anline structures bonded through the nitrogen to surface boron atoms and new B-OH groups formed. Surface boron impurity acted as an adsorption and dissociation center.Canadian Journal of Chemistry, 47, 1281 (1969) Recent studies have provided evidence that much of the reactivity of porous glass surfaces could be attributed to the presence of impurities (1-7). With NH, sorption on highly dehydrated porous glass (4), infrared spectra indicated that residual boron played a role in physical adsorptions, dissociative chemisorption, as well as in an undissociative chemisorption involving the coordination of NH, to boron present on the glass. The surface =B:NH, complex was weakly bonded, but may have been the precursor of dissociated, more tightly held species. In order to determine if similar surface complexes were formed with molecuies larger than NH,, we have studied the sorption of aniline on highly degassed, deuterated, and fluoridated porous glass as well as on pure and boria-impregnated silicas. ExperimentalThe glass adsorbents were 10 x 25 x 1 mm pieces of Code 7930 porous glass purchased from Corning Glass Works. Silica and 3 % B,O, -Si02 samples were prepared from Cab-0-Sil silica (G. Cabot Co., Boston, Mass.). Most experimental details including the dehydration, deuteration, and fluoridation of specimens have been described elsewhere (1-5). Reagent grade (Matheson Co.) aniline was dried over Na2S04 and distilled 4 times; the final purification was done by repeated freezing and thawing in vacuo. A thermocouple gauge was used to measure the pressure of adsorbate. Spectra were recorded with Perkin-Elmer models 521 and 621 spectrophotometers. The ordinates of spectra shown in the various Figures are displaced to avoid overlapping of traces. The number at the left of each spectrum shows the % transmittance for the particular spectrum at 4000 cm-l. ResultsTrace A of Fig. 1 is a typical "background" spectrum of highly-degassed porous glass; the prominent features are a band at 3748 cmp' due to surface silanols and a smaller but sharp band at 3703 cm-' due to surface B-OH groups (1-5), hereafter termed SiOH and B-OH bands. When such a glass specimen was exposed to aniline vapor at room temperature at increasingly higher pressures from 10 to 200 p and was then degassed, the surface hydroxyl bands were altered and a series of absorptions caused by sorbed aniline were observed. A typical sorption-desorption cycle is illustrated by the alphabetical sequence...